The broad aim of this proposal is to examine pathologic mechanisms in congenital myasthenic syndromes (CMS). The CMS are highly disabling disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanism(s). The CMS are not uncommon but are seldom diagnosed or treated correctly. Appropriate morphologic and electrophysiologic analysis, however, can identify the defects involved. Furthermore, such studies can implicate a candidate mutant protein or gene; for example, the electrophysiologic identification of a kinetic abnormality of AChR is presumptive evidence for a mutation in an AChR subunit. The approach to the CMS will be through five steps: (1) clinical assessment, including electromyography and tests for anti- AchR antibodies; (2) morphologic assessment, including cytochemical or immunocytochemical localization of acetylcholinesterase, AChR, and AChR subunits at the endplate (EP), estimate of the number of AChR per EP, quantitative ultrastructural analysis of the EP, and evaluation of the density and distribution of AChR on the junctional folds; (3) electrophysiologic assessment, consisting of conventional microelectrode studies of EP potentials and currents, estimates of parameters of quantal release, and evaluation of AChR channel kinetics through noise analysis and single channel patch-clamp recordings; (4) mutational analysis of AChR subunits when a kinetic abnormality of AChR is recognized; and (5) collaborative expression studies using genetically engineered mutant AChR when an AChR subunit mutation is identified. The proposed studies are important for diagnosis, treatment and prevention of the CMS; and for gaining additional insights into synaptic function and structure-function relationships of AChR. Moreover, recognition of spontaneous mutations in the EP nicotinic AChR should spur the search for mutations in neuronal nicotinic AChRs and other ligand-gated channels in various neurologic and psychiatric disorders.